1. Field of Invention
The present invention relates generally to the field of high frequency antennas, and more particularly to the field of high-gain, multi-dipole array antennas constructed using inexpensive manufacturing techniques.
2. Discussion of Background
The U.S. Federal Communications Commission (FCC) allocates a certain number of frequency bands where a license is not required for use. For example, many garage-door openers operate in the unlicensed 49 MHz band. Similarly, the unlicensed 2.4 GHz frequency band has become popular for connecting computers to a wireless LAN.
Unfortunately, the 2.4 GHz band available in the U.S. and worldwide hosts a myriad of devices and competing communications standards that have led to increasing interference and degraded performance in the wireless networking world. Devices operating at 2.4 GHz include common household items such as microwave ovens, cordless phones and wireless security cameras, in addition to the myriad computing devices that are wirelessly networked together. To add to the confusion, the industry has deployed multiple standards for wireless networking at 2.4 GHz. The IEEE 802.11b standard is, as of the filing date hereof, most commonly used for enterprise wireless LANs. The Home RF standard also exists for wireless LANs in the home, and Bluetooth has been developed as a short-distance wireless cable replacement standard for short-range, low-rate applications.
The interference and performance issues at 2.4 GHz have the wireless LAN industry headed for the open 5.15 to 5.35 GHz frequency band, where the opportunity exists for a much cleaner wireless networking environment. The allocated unlicensed 5 GHz band is devoid of interference from microwave ovens and, in the U.S., provides more than twice the available bandwidth of the allocated unlicensed 2.4 GHz band, thereby allowing for higher data throughput and more simultaneous users, and the potential for multimedia application support. This open 5 GHz spectrum provides an opportunity for the potential creation of a unified wireless protocol that will support a broad range of devices and applications. Everything from cordless phones to high-definition televisions and personal computers can communicate on the same multipurpose network under a single unified protocol. As a result, an antenna operating in the unlicensed frequency band above 5 GHz would encourage the creation and support of a wide range of low and high data rate devices that could all communicate on a single wireless network.
As to antenna design to take advantage of the above described opportunity for high-frequency wireless communication, the industry""s foremost objective is to provide antennas having (1) the lowest possible manufacturing costs with consistently uniform performance, (2) high gain, (3) high directivity when desired, and (4) design characteristics that can be applied in both the current majority-used frequency bands (such as 2.4 GHz) and the newly utilized bands (particularly between 5 GHz and 6 GHz).
Conventional dipole antennas (also commonly known as Franklin antennas), in which each member of a pair of fractional wavelength radiators are fed in anti-phase, produce a substantially omni-directional radiation pattern in a plane normal to the axis of the radiators. However, providing such an omni-directional structure on a substantially planar (and inexpensively produced) surface, such as a printed circuit board substrate, has proven a challenge. Existing attempts to achieve such planarity and performance rely on vias that penetrate the substrate to interconnect a plurality of conducting planes, thereby adding substantially to the cost of the antenna.
U.S. Pat. No. 5,708,446 discloses an antenna that attempts to provide substantially omni-directional radiation pattern in a plane normal to the axis of the radiators. The patent discloses a corner reflector antenna array capable of being driven by a coaxial feed line. The antenna array comprises a right-angle corner reflector having first and second reflecting surfaces. A dielectric substrate is positioned adjacent the first reflective surface and contains a first and second opposing substrate surfaces and a plurality of dipole elements, each of the dipole elements including a first half dipole disposed on the first substrate surface and a second half dipole disposed on the second substrate surface. A twin line interconnection network, disposed on both the first and second substrate surfaces, provides a signal to the plurality of dipole elements. A printed circuit balun is used to connect the center and outer conductors of a coaxial feed line to the segments of the interconnection network disposed on the first and second substrate surfaces, respectively.
However, in order to connect the coaxial cable to the interconnection network, U.S. Pat. No. 5,708,446 requires a via to be constructed through the substrate. This via""s penetration through the substrate requires additional manufacturing steps and, thus, adds substantially to the cost of the antenna.
Furthermore, other attempts require branched feed structures that further increase the number of manufacturing steps and thereby increase the cost of the antenna. A need exists to use fewer parts to assemble the feed so as to reduce labor costs. Present manufacturing processes rely on a substantial amount of human skill in the assembly of the feed components. Hence, human error enters the assembly process and quality control must be used to ferret out and minimize such human error, which adds to the cost of the feed.
Such human assembled feeds also provide inconsistent performance. For example, U.S. Pat. No. 6,037,911 discloses a phased array antenna comprising a dielectric substrate, a plurality of dipole means, each comprising a first and a second element, the first elements being printed on the front face and pointing in a first direction and the second elements being printed on the back face, and a metal strip means comprising a first line printed on the front face and coupled to the first element and a second line printed on the back face and coupled to the second element. A reflector means is also spaced to and parallel with the back face of the dielectric substrate and a low loss material is located between the reflector means and the back face, whereby the first and second lines respectively comprise a plurality of first and second line portions and the first and second line portions respectively being connected to each other by T-junctions. However, in order to provide a balanced, omni-directional performance, U.S. Pat. No. 6,037,911 requires a branched feed structure through the utilization of T-junctions. These T-junctions add complexity to the design and, again, increase the cost of the antenna.
To address the shortcomings of the available art, the present invention provides a planar antenna having a scalable multi-dipole structure for receiving, and transmitting high-frequency signals, including a plurality of opposing layers of conducting strips disposed upon either side of an insulating (dielectric) substrate.
In one embodiment, the present invention is an antenna in which each dipole is bifurcated along a horizontal axis, with one half of a dipole disposed on one side of a substantially planar insulating layer and the other half disposed on the other side of the insulating layer. Additionally, each dipole half is in electrical communication with a feed structure independent of its other half, and a plurality of dipoles are preferably dispersed symmetrically along the feed structure.
In another embodiment, the present invention is an antenna that is optimized to function between 5.15 and 5.35 GHz, preferably with a center frequency of 5.25 GHz. In an alternative, higher gain embodiment of the present invention, a plurality of dipoles is vertically integrated along the feed structure to create a serial, co-linear antenna.
Advantages of the present invention include: provision of a highly effective dipole structure in an inexpensive, printed implementation (printed radiating elements on opposing sides of a planar, insulating substrate); the integration of a balun with an antenna feed on a planar substrate; and, provision of a feed line and feed line branches to each of a plurality of radiating elements such that an excellent impedance match is obtained over a wide frequency range. Also, the inventive antenna""s lack of vias and inclusion of balanced, independent feed structures significantly reduces system design time, manufacturing costs and utilized materials. Preferably, cost is further minimized through the use of standard manufacturing processes and eliminating the introduction of human error.